It is known from the state of the art how to project information in the field of sight of a user, such as, for example, a driver or a pilot, by means of a head-up display. When speaking of a head-up display, which is also abbreviated as HUD, one is to understand a display system wherein the user can essentially maintain the position of the head or alternatively the viewing direction in the original orientation to be able to view the displayed information. Such head-up displays generally feature their own picture generating unit, which provides the information to be represented in the form of an image, an optical module, which permits the beam path through the head-up display to the output opening and which is also referred to as mirror optics, as well as a projection surface, for the display of the image that is to be generated. The optical module guides the image onto the projection surface, which is designed as a reflective, transparent disc and is also referred to as a combiner. In a special case, a windshield that is appropriate for this use is employed as a projection surface. The vehicle driver contemporaneously sees the mirrored information of the picture generating unit and the actual surroundings behind the windshield. The attention of the vehicle driver, for example, when driving a motor vehicle, therefore remains focused on that which is occurring in front of the vehicle, while they are able to grasp the information that is projected in the field of vision.
Among the information to be presented is the display of operational conditions, such as indications relating to speed, engine speed, cargo load, equipment, as well as information from navigation systems, radar systems or homing or sighting systems. Beyond this, for example, in motor vehicles, it is also possible that information relating to currently valid distance limitations, such as prohibition of passing and speed limits, or the setting of adaptive cruise control, abbreviated as ACC, and other things can also be made to be displayed.
The use of a so-called contact analog display, in 2D or 3D perspective, in vehicles is also known. When speaking of contact analog information or display elements, one should understand information or display elements which can be shown in the current view of the vehicle driver that is positionally and directionally correct, in such a way that the vehicle driver is given the impression that this information or display elements are a component of the surroundings of the vehicle. It is thus that the impression is given, for example, that an arrow that is represented, which indicates a change in driving direction or lane change, is directly depicted on the road surface, inasmuch as the pictorial depiction of the arrow has been superimposed on the surroundings perceived by the vehicle driver. This total or partial concealment or superimposition of such positionally and directionally correct overlay information thereby leads to a supplementation of the image that is perceived by the vehicle driver of their surroundings.
Beyond representing a navigation arrow that appears directly on the road, it is also, for example, possible that a distance bar is displayed, which is meant to display the distance to be maintained from the preceding vehicle. It is also possible that a representation of walls and barriers are placed upon existing road markings.
So-called holographic optical elements or components are known from the state of the art, the holographic characteristics of which are used for the optics of devices. Using the same elements exhibiting a hologram, it is possible to replace traditional lenses, mirrors and prisms.
The simplest known holographic-optical component is the so-called Fresnel zone plate, which is also called a zone plate due to its characteristics. A zone plate is the hologram of a point and therefore contemporaneously functions as a transmission hologram as well as a converging lens when the true image is observed, and as a diffusion lens when one starts from the virtual image. Elements of this type have special characteristics, such as, for example, selectivity of color and the angle of incidence of light. It is possible, for example, that the components refract light for a particular angle of incidence, and be completely transparent for another angle of incidence.
Designs of the holographic-optical elements with varying diffraction of the light, dependent on its wavelength, allow for a break down in colors of the spectrum, in the same way as with a prism.
There are solutions that are known, whereby planar mirrors, concave mirrors or convex mirrors are produced with the help of reflection holograms, which are able to reflect light in such a way that the angle of incidence differs from that of the angle of reflection. Elements of this type are, for example, employed to guide daylight towards a photovoltaic system or into the interior of a space.
A display device, a vehicle with a display device, and a computer program product for a display device, are known from DE 102014000487 A1. This display device comprises at least a first concave mirror and a second concave mirror, whereby the second concave mirror exhibits at least one opening, a convex cavity that spans both concave mirrors, a diffracting optical element that is arranged in the cavity with a multitude of optical phase modulation cells, whereby the diffractive optical element provides a hologram. On top of this, at least one light source to illuminate the phase modulation cells of the diffractive optical element and a transparent touchpad, that covers the at least one opening and foresees an input of data, whereby the diffractive optical element is arranged in the cavity in such a way that the beam that exits the at least one light source is modulated by the phase modulation cells, passes through the opening and reproducing a holographic image on top of the transparent touchpad within a defined field of vision, by means of the transparent touchpad.
A holographic screen is known from DE 19730563 A1, which appears black, grey or colored in ambient light and primarily finds use in a motor vehicle. It is disclosed that the screen is arranged on a slightly angled surface of the roof liner in the middle to front area of the vehicle. After the at least one projection system has been switched on, the screen is illuminated either by laser scanning or flatly by means of an appropriate light source, such as, for example, a halogen light. The laser thereby gets modulated according to the desired or required image information or the flat projection overlays the image content by means of light valves such as slides or a transparent small LCD screen. It is possible to use three colors (RGB) (red, green and blue) for a full color representation.
In the event in which the modulated projection beam was to reach the screen as a black projection surface, from the point of view of the viewer, a clear, high-contrast image results at a viewing distance that doesn't necessarily need to be identical to the plane of the projection surface. This viewing distance can therefore also be represented at a greater distance from the eye, whereby the accommodation of the eye can be facilitated. It is furthermore possible to realize a three-dimensional representation.
Using a known head-up display (HUD), it is possible to generate a virtual image in a plane with a determined distance to the viewer. In the case of need for a representation of two or multiple virtual images, which are to be represented at varying distances or planes, it is necessary to employ two or more picture generating units. The problem with such a solution is a greatly increased need not only in the installation space required in the vehicle in the dashboard area, but also in the costs of two separate picture generating units.